Image scanners convert a visible image on a document or photograph, or an image in a transparent medium, into an electronic form suitable for copying, storing or processing by a computer. An image scanner may be a separate device or an image scanner may be a part of a copier, part of a facsimile machine, or part of a multipurpose device. Reflective image scanners typically have a controlled source of light, and light is reflected off the surface of a document, through an optics system, and onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. Transparency image scanners pass light through a transparent image, for example a photographic positive slide, through an optics system, and then onto an array of photosensitive devices.
In general, there is need for reducing the cost of an image scanner. One expensive component is the optics system used to focus a line on a document onto an array of photosensors. However, some low cost lens designs are sensitive to environmental changes, particularly temperature and mechanical movement. In particular, some low cost plastic lenses are more sensitive to temperature than more expensive optical glass lenses. It is common for an image scanner to include a lamp that radiates substantial heat. Copiers and multi-purpose devices may also include heaters for fusing toner onto paper or for drying ink. Before scanning is requested, lamps and heaters may be off or in a low-power standby condition. During scanning, the internal temperature of the image scanner may change substantially. In addition, it is common to move the optics system of a scanner during a scan. With changing temperature, and mechanical movement, the magnification of an optics system may change slightly during a scan. If magnification changes during a scan, lines or edges, parallel to the direction of scanning, that are straight in the image on the document may appear curved in the resulting scanned image.
Scanners typically use linear arrays of photosensors. Sensitivity varies from photosensor to photosensor. It is common to calibrate the sensitivity of each individual photosensor before scanning by scanning a calibration strip having a uniform reflectance. This calibration also compensates for non-uniform illumination, and may also compensate for small obstructions or imperfections in the optical path, such as dust or a fingerprint on a lens or mirror. Assume, for example, that a small group of individual photosensors receives light during pre-scan calibration that is partially obscured by a dust particle on a lens. If magnification changes, the group of photosensors affected by the dust particle changes. An individual photosensor that was properly calibrated before scanning may be inappropriately calibrated after magnification changes. As a result, streaks may appear in the resulting scanned image.
There is a need for real time detection of and compensation for magnification change.